The surfactants present in the aqueous dross liquor (usually called "supernate") can come from two sources. One source is the anionic surfactant or dispersant used during the polymerization of tetrafluoroethylene. This surfactant is an ammonium perfluoroalkanoate (called AMPFAk hereafter). A preferred surfactant of this type is ammonium perfluorooctanoate (AMPFO hereafter).
The second source is the surfactant used in concentrating the raw dispersion obtained from the polymerization. This surfactant is a nonionic one and is a hydrocarbyl oxyethoxylate. Subclasses are aliphatic alcohol ethoxylates and alkyl phenol ethoxylates. A preferred surfactant of this type is "Triton" X-100. These surfactants are sometimes generically called hydrocarbyl oxyethoxylated polyethers.
The raw dispersions are typically concentrated to 55-60 wt. % solids by a so-called "thermal concentration" process such as that described in U.S. Pat. No. 3,037,953. The raw dispersion, typically containing from 35 to 45 wt % solids, is charged to a jacketed vessel capable of being heated to temperatures below the boiling point of the dispersion. To the raw dispersion is added a nonionic surfactant of the hydrocarbyl oxyethoxylate type. Preferably, ammonium hydroxide is also added to render the dispersion alkaline (pH=9 to 11) to accelerate the thermal concentration process and to resist later bacterial "souring" of the concentrated dispersion product.
The contents of the vessel after mixing are then heated to a temperature somewhat above the cloud point of the surfactant, and allowed to stand (unagitated) at this temperature until separation into two liquid layers is essentially complete. The nearly clear supernatant layer (hereafter called "supernate") is then syphoned or decanted off, and the lower layer, consisting of a concentrated, colloidal dispersion of the polymer containing 55-60% solids by weight, is pumped to storage tanks from which it is withdrawn for subsequent packaging and commercial sale.
A preferred nonionic hydrocarbyl oxyethoxylated surfactant for use in the thermal concentration process is an alkyl phenol ethoxylate, namely, t-octylphenol ethoxylate containing, on average, from 9 to 10 ethylene oxide units per molecule. Such a surfactant is commercially available as "Triton" X-100, sold by the Rohm & Haas Company. A suitable nonionic surfactant of the aliphatic alcohol ethoxylate type is "Tergitol" 15-S-9, sold by Union Carbide Corporation. This surfactant contains from 11 to 15 carbon atoms per molecule of aliphatic alcohol and, on average, 9 moles of ethylene oxide units per mole of alcohol.
The surfactants present in the supernate formed in the thermal concentration process are intrinsically valuable, but their recovery is difficult because their concentrations in the supernate are relatively low, especially for the more valuable surfactant, AMPFAk. Furthermore, the surfactants consist of molecules having more-or-less distinct polar and non-polar portions, so that they tend to "bridge" ordinarily immiscible media such as might otherwise be used for their separation, as, for example, by liquid-liquid extraction. In addition, both surfactants have pronounced, and generally undesirable, foaming tendencies, especially in aqueous media.
Despite these difficulties, recovery of the surfactants is desirable because the disposal of the supernate by acceptable means is difficult. Ethoxylates, represented by "Triton" X-100, though considered biodegradable, degrade only slowly, even by bacteria acclimated to feeding upon it, and AMPFO is not biodegradable, nor is PTFE. The foaming tendency of the supernate can also pose troublesome problems in its disposal.
A known method for recovering "Triton" X-100 from dilute aqueous solutions in the concentration range typical of that found in supernate is to take advantage of the "inverse" solubility--temperature relationship found in the "Triton"--water system. Thus, such solutions, homogeneous at room temperature, can simply be heated to temperatures between the cloud (ca. 70.degree. C.) and normal boiling points (ca. 100.degree. C.) to effect a separation into two liquid phases, one rich in "Triton" and the other poor in "Triton". However, the basic effect of AMPFAk on the "Triton"--H.sub.2 O system is to promote compatibilization of these components; i.e., to increase their miscibility. Thus, the effect of the presence of AMPFAk is to increase both the solubility of water in the "Triton"-rich phase, and the solubility of "Triton" in the water-rich phase; the cloud point (the temperature at which miscibility becomes limited) is raised as a result. AMPFAk, at the concentration level at which it is normally present in supernate, prevents a phase separation from being obtained by simply heating the supernate to a temperature up to as much as about 10.degree. to 20.degree. C. above that used in the thermal concentration of PTFE dispersion (but below the normal boiling point of the supernate).
Another limitation of this "thermal" phase-separation method is that, in the case where AMPFO (or its corresponding acid, usually designated C-8 acid) is present, the fluorosurfactant does not become well separated from the nonionic surfactant, even if phase separation does occur. On the contrary, the usual tendency is for the majority of the fluorosurfactant present to remain in the phase which is richer in the nonionic surfactant.
Various other recovery procedures have been devised. The use of ion-exchange for the recovery of fluorinated surfactants from aqueous media has been disclosed in the patent literature, as, for example, in U.S. Pat. No. 3,882,153, and more recently in U.S. Pat. No. 4,282,162. However, the treatment of the above-described supernate by ion exchange, using the same resin ("Lewatit" MP-62, a weakly basic anion exchanger sold commercially by Bayer AG) predominantly employed in the Examples of U.S. Pat. No. 4,282,162, and following the absorption and elution procedures of that patent (using an aqueous HCl/n-propanol mixture for elution), did not produce satisfactory long-term results. The ion-exchange resin on repeated use rapidly lost its capacity for AMPFO, so that after five cycles of use it retained only a small fraction of its initial capacity. The cause of this drop-off could not be readily ascertained. In addition to this major deficiency in performance, the ion-exchange resin is relatively expensive unless it can be reused many times, and the overall procedure is cumbersome for practical application. With the nonionic surfactant present, extensive washing and rinsing of the resin is necessary for its removal. Furthermore, the presence of the dispersed PTFE in supernate is also a potential source of fouling of the ion-exchange resin, and the other anionic components (besides AMPFO) present may be capable of reducing its exchange capacity for AMPFO.
It is desirable to provide a process for recovery of the surfactants discussed above which employs common, inexpensive inorganic reagents and a simple organic solvent, thereby avoiding the complexity of such techniques as ion exchange for effecting the separation of the surfactants, which overcomes foaming by appropriate choices of media and conditions, especially of pH level and which can be carried out at atmospheric pressure and at ambient temperature.